On Thu, 3 Aug 2017 10:18:58 -0700 (PDT), wrote:

This old thread but applies now with 2017 eclipse...

Someone says the numbers DO add up so I figure using two number 10s
from cheap Harbor Freight welding goggles will be okay.

they do add but not linearly. The sum is usually one less than the
numerical sum.

I have a wide variety of welding filters here in the lab because of
the wide variety of materials I have to induction heat in a demo,
photograph the heating and send the results to a potential client.

I've built a mechanism to hold a stack of filters in front of my
camera and have installed open source software on the camera to
provide an intervalometer function. The combination of filters that
make both my eyes and my camera happy are a #6 and a #12. According
to a chart I have, that's equivalent to a #15 filter.

Your stack of 2 #10s will probably be too dense, as my chart shows
them to be equivalent to a #18. Especially if you're photographing
the event, you want your camera to be stopped down and timed out
almost completely so that it'll have enough range to see the corona
and solar flares during totality.

Even when just watching, a stack of 2 or 3 filters will be almost
essential. During the last eclipse I used a welding hood with a #13
filter. When totality happened, I flipped up the filter and watched
unfiltered. When that first little bit of the sun shown at the end of
totality, my eyes were instantly dazzled. They teared up and I saw
spots for most of the rest of the event. I wish I'd had a #2 or #3
filter to watch totality with.

While we're on the topic, I'll say something about the cheap cardboard
glasses being sold or given away. I got 2 sets. I drove to my
photographing place, laid down in the back of my truck, put the
glasses on, opened my eyes and... Lasted for about a minute. The
filter is dense enough but the surround is so narrow that glare from
my cheeks reflecting off the back side of the filter dazzled me.

So I've made up a couple of shields out of poster board that will
protect my whole face from the glare.

So any of y'all planning on using those cheap cardboard glasses need
to do a dry run with full sunlight to see if you can stand the glare.

John


John DeArmond
http://www.neon-john.com
http://www.tnduction.com
Tellico Plains, Occupied TN
See website for email address